Integrated circuitry requires deposition of electronically conducting channels through which electronic communication between active and passive components then takes place. The dimensions of electronic conducting channels vary widely depending on device requirements. In microchip technology, such dimensions are micron and submicron, whereas in printed board circuitry much larger dimensions can be tolerated. Lithography is the driving force for both the macro- and micro-scale integrated circuit technology (L. F. Thompson, C. G. Wilson, and M. J. Bowden, Eds., "Introduction to Microlithography", ACS Symposium Series 219, Washington D.C., 1983).
Commercially, the fabrication of conducting channels is accomplished using one of two standard multi-step photolithographic techniques. In the first, a 1 to 2 .mu.m thick film of photosensitive polymeric material (photoresist) is deposited, usually by spin casting, on top of an inert substrate. Baking of the polymer is often required to improve film characteristics. Two types of resist are available, those which dissolve at a decreased rate following exposure to UV or visible light (negative resists), and those which exhibit an increased rate of dissolution (positive resists) following exposure to UV or visible light.
Initially, a relief image is formed in the photoresist by selective exposure to UV irradiation through a photomask. Pattern development is achieved by dissolution of the exposed or unexposed resist. Metal is then vapour deposited over the whole surface. The bare and exposed surface receives a layer of metal whereas those regions still covered by polymer resist material do not. The formation of isolated electronically conducting channels is achieved by removal of the remaining polymer resist.
In the second method, the metallic layer is deposited onto the inert surface prior to casting the polymer resist layer. Following baking, the resist is exposed through a photomask, and the exposed or unexposed photoresist is removed by dissolution. Etching of the exposed metal and removal of the remaining polymer resist renders the metallic conducting pattern. In both methods, fabrication of metallic structures on surfaces is time consuming, and costly, due to the large number of intricate procedures involved.
An alternative process to photolithography involves direct laser write technology wherein a monochromatic laser beam of controlled dimensions is focused upon, and scanned across the surface of the polymer resist film. The image generated in the film is a copy of the path scribed by the laser beam. Computer aided design is employed to fabricate structures useful for electronic devices. In the context of lithography, laser beams replace and are analogous to the UV lamp/photomask arrangement, but all other steps in the fabrication of conducting channels are virtually the same.
U.S. Pat. No. 5,109,149, Leung, issued Apr. 28, 1992, is directed to a laser, direct-write system for making personalized custom or semi-custom integrated circuits with a very fast turnaround time. The system includes a method and apparatus for high precision scanning of a submicron laser spot. The laser beam is scanned at the entrance of a beam expander. The beam expander reduces the scan angle and error produced by a mechanical scanning device such as a rotating polygonal mirror. The smaller scan angle at the output of the beam expander matches well with the projection optics of a laser, direct-write semi-custom integrated circuit production system. The scan error reduction permits more accurate positioning of the focussed laser spot on the surface of the semi-custom integrated circuit.
Japanese Patent Application No. 88232654, Kokai, involves formation of a conducting polypyrrole pattern by irradiating a polymer film consisting of pyrroles, electrolyte, redox polymer. Irradiation causes a redox reaction resulting in polymerization of pyrrole to a conducting form.
The present invention has the following significant distinctions from Japanese Patent Application No. 88232654:
1. The concept of image formation in the present invention is photo-crosslinking and insolubilization. In Japanese Patent Application No. 88232654, it is a photopolymerization. PA1 2. The present invention requires one component. No other components are present or necessary. Japanese Patent Application No. 88232654 requires monomer, electrolyte and redox polymer. PA1 3. The conductivity of the image in the present invention can be readily controlled. In Japanese Patent Application No. 88232654, it cannot. PA1 4. The present invention involves a solid state reaction and image processing is totally compatible with present lithographic technologies. High resolution (micron) can be obtained. The same cannot be said for the Japanese patent. PA1 5. The present invention utilizes preformed polymers which may be varied and controlled. The Japanese patent utilizes pyrrole monomers. The properties of the resultant polymer cannot be controlled.
U.S. Pat. No. 4,962,158, Oct. 9, 1990, Kobayashi et al., discloses a radical polymerizable composition comprising (1) a compound having a pi-electron conjugated structure and (2) a radical polymerizable compound, which is useful for molding into an arbitrary shape, which can be rendered electrically conductive and which is therefore useful as a material for electrodes or circuits in the electrical and electronic industry.
Specifically, a composite comprising a conducting polymer and a polymerizable free radical compound is rendered insoluble by a free radical initiator. When the initiator is activated by light, this leads to photoinsolubilization of the polymerizable free radical compound. The insolubilization step involves the polymerizable component, not the conducting polymer.
U.S. Pat. No. 5,137,799, Aug. 11, 1992, Kaempf et al., discloses an electrically conductive resist material comprising (1) at least one polymer which is sensitive to ionizing radiation and (2) a soluble electrically conductive oligomer or polymer. A process for producing the resist material is also described, comprising admixing an electrically conductive oligomer or polymer dissolved in a solvent to at least one polymer which is sensitive to ionizing radiation. The resist material is useful in preparing electron beam resists which prevent electrostatic charging and resultant electrostatic fields.
When the radiation sensitive polymer, namely, poly(methylmethacrylate), is irradiated, chain scission of that polymer occurs and the mixture is therefore rendered more soluble. The conducting polymer is not affected. As with the process disclosed in Kobayashi et al., the key component in Kaempf et al. is the reaction of the non-conducting polymer, not the conducting polymer. In this sense, Kaempf et al. are very similar to Kobayashi et al. However, in Kaempf et al., the non-conductive polymers are degraded by irradiation rather than being cross-linked. As a result of the conductive polymer being only a minor component of the combination in Kaempf et al., the conductivities of the Kaempf et al. system are not very large.
Neither Kobayashi et al. nor Kaempf et al. teach that a pi-conjugated polymer can be insolubilized by cross-linking and that the cross-linked polymers can be rendered electrically conducting by oxidation.
Strategies for obtaining chemically related polymers possessing superior electronic conductivity, environmental stability, processability, and synthetic efficacy have been actively pursued ever since the discovery that polyacetylene can be oxidized to yield materials of high electronic conductivity. With few exceptions, it is the oxidized form of .pi.-conjugated polymers which have received most attention. Unfortunately, many of them are unstable and revert back to their neutral insulating state too quickly for practical use. One such class of polymer, poly(3-alkylthiophenes), while possessing high coductivity and good processability, loses its conductivity in a matter of hours.